Controlled Formation of Native Defects in Ultrapure ZnO for the Assignment of Green Emissions to Oxygen Vacancies

Abstract

Ultrapure ZnO nanopowders were synthesized via vapor-phase-based methods under oxygen-deficient conditions. The type and relative proportions of intrinsic point defects were studied by photoluminescence (PL) and EPR spectroscopies performed under strictly controlled conditions. Besides coupled PL/EPR signals recently assigned to Zni+ (2.80 eV, g = 1.96), two green emissions were systematically detected at 2.50 and 2.22 eV without EPR counterparts, whereas their contributions were observed to depend on the synthesis oxygen partial pressure (PO2). Among diamagnetic defects likely to be formed in O2-poor conditions, Zni0 and Zni2+ were discarded based on their energy levels that were reported to rather correspond to the transitions associated to match the violet light. Conversely, the involvement of oxygen vacancies (VO0 and VO2+) as recombination centers for the green emission in ZnO was supported by Raman and XPS data. In line with the expected trends based on formation energies, the always dominant green luminescence (2.50 eV) was assigned to VO2+ and the weaker one (2.22 eV) to VO0. The involvement of an electron-containing defect (VO0) was confirmed by visible light absorption observed in DR UV–vis spectra. We also showed that the Zni+/VO2+ ratio can be tuned by PO2 or by the choice of static or flow synthesis conditions. Overall, this study demonstrates that by controlling the conditions during synthesis, processing, and spectroscopic investigations, the ultrapure ZnO nanopowders represent reliable models for the identification of photoluminescent crystal defects—an approach that can be widely applied on other systems.